Austenitic-ferritic steel of the superduplex type applicable to the fabrication of seamless tubes

ABSTRACT

This steel is applicable for making seamless tubes with a high mechanical strength, high pitting resistance and a high hot-formability, by means of a process comprising meltdown in an electric-arc furnace, refining in an AOD converter, ascensional ingot casting and Gothic section billet rolling, and then transforming the billet obtained into seamless tubes by upsetting the gothic billet into a round followed by hot piercing in a vertical press, tube extrusion in a horizontal press, reduction in a stretch reduction mill and Pilger cold-rolling, and having a composition of not more than 0.02% C, 0.65 to 0.85% Mn, 0.40 to 0.60% Si, 25.2 to 25.6% Cr, 6.2 to 6.6% Ni, 3.6 to 3.8% Mo, 0.24 to 0.30% N, not more than 0.025% P, not more than 0.002% S, with no Cu and W, and with the addition of a percentage of B ranging between 0.0015 to 0.0030% and a percentage of Ca ranging between 0.0010 to 0.0050%.

This application is a 371 of PCT/ES96/00201 dated Oct. 29, 1996.

OBJECT OF THE INVENTION

The invention relates to superduplex austenitic-ferritic stainlesssteel, applicable in means requiring materials having a high mechanicalstrength, high pitting resistance, good intergranular corrosionbehaviour and a high hot-formability to allow tubes to be made withouthot- or cold-welding.

BACKGROUND OF THE INVENTION

Austenitic-ferritic stainless steel alloys are well-known since the1930s and 1940s and are essentially based on compositions with a highchromium, nickel and molybdenum content. Reference could in this sensebe made to U.S. Pat. No. 2,432,616, applied for in 1945 by FRANKS etal., which relates to alloys with a high mechanical strength at hightemperatures, one of the alloys disclosed consisting of not more than0.35% carbon, not more than 2.0% manganese, not more than 1.0% silicon,2-40% nickel, 10-30% chromium, 1-5% molybdenum, not more than 0.25%nitrogen, 0.5-5-0% tungsten and 0.1-0.7% boron.

Steels of this kind have evolved over the years, changing theirstructure to adapt to the various needs required of materials as regardsmechanical strength, corrosion resistance, good structural stability andthe like. In this sense, duplex and superduplex steels are well-knownand used. Duplex (ferritic-austenitic) steels have a composition with achromium percentage of 22%, 5% nickel, 3% molybdenum and 0.17% nitrogen,which gives them a good corrosion resistance in marine and similarenvironments. The composition of superduplex steels, on the other hand,is 25% chromium, 6.8% nickel, 3.7% molybdenum and 0.27% nitrogen, toreach a rate of pitting resistance in excess of 40. The increasedpercentages of chromium, nickel and molybdenum used over duplex steelsprovides such enhanced properties as mechanical strength and corrosionresistance.

It is moreover normal for the standard compositions of duplex andsuperduplex alloys to include other elements, albeit in smallerquantities, such as manganese, sulphur, silicon, copper, tungsten,magnesium, aluminium, rare earths and the like.

Reference can in this sense be made to a number of duplex or superduplexPatents in which some of the components vary over the usual standards inorder to enhance a particular characteristic thereof, generallyassociated with a specific application of the resultant steels, namelyfor instance:

GB Patent 2 160 221 to NIPPON KOKAN K.K. relates to aferritic-austenitic stainless steel for making seamless tubes with anenhanced impact toughness, therefore disclosing the addition of solublealuminium in a quantity not exceeding 0.02%. The other components lieapproximately within the usual ranges of ferritic-austenitic steels ofthis kind.

U.S. Pat. No. 4,604,887 to OHTSUBO et al. relates to a manufacturingmethod which avoids surface defects, along with flaws at the head andtail of the tubes, due to special rolling conditions and a specialchemical composition, which is essentially distinguished from otherduplex steels by a high copper content and the addition of smallquantities of aluminium.

European Patent 0 339 004 to SANDVIK AG relates to anaustenitic-ferritic stainless steel which includes tungsten, vanadiumand cerium in the alloy to provide the resultant steel with highresistance in chloride ion environments and preferably applicable formanufacturing medical implants. A steels of similar characteristics isalso described in European Patent 0 220 141 to SANTRADE LTD.

GB Patent 2 203 680 to NIPPON YAKIN KOGYO CO. LTD. relates to a processfor continuously making ferritic-austenitic stainless steel, to whichend considerable quantities of tungsten and vanadium are added, alongwith very small quantities of boron, the other elements lying within theusual percentages.

European Patent no. 0 545 753 to SUMITOMO METAL INDUSTRIES LTD. relatesto a superduplex steel of high mechanical strength and corrosionresistance, albeit with a very reduced tendency towards intermetallicprecipitation, the main characteristic of the alloy disclosed being theaddition of higher than usual tungsten percentages, for the quantitiesof tungsten and chromium may thus be reduced and consequently so may thenegative effects of these elements as regards the precipitation of theintermetallic compounds be minimised. Small quantities of other elementssuch as vanadium, calcium, magnesium, boron or rare earths are alsoadded to improve the hot-ductility of the resulting steel.

European Patent no. 0 594 935 to CENTRO SVLUPPO MATERIALI S.p.A. relatesto a stainless steel alloy and the respective process for obtainingseamless tubes, based on the aforesaid alloy, useful in acidenvironments, namely for instance in oil exploration. The alloy subjectof this invention is essentially characterised by the addition ofgreater than usual tungsten and copper percentages, in order to improvethe mechanical and corrosion resistance properties.

European Patent no. 0 566 814 to FORONI S.p.A. relates to a superduplexsteel with a high corrosion resistance in an acid or alkalineenvironment, for which purpose greater than usual quantities of copperand tungsten are added.

DESCRIPTION OF THE INVENTION

The present invention relates to a superduplex steel alloy that retainsits mechanical strength and corrosion resistance characteristics and ischaracterised by having a high ductility which allows tubes to berapidly, simply and cheaply both hot- and cold-formed. Two elements aretherefore added which are not usual in standard steels of this kind,namely boron and calcium, in suitable proportions in order to achievethe required ductility, moreover optimising the percentages of the otheralloy components in order to retain a good mechanical strength and ahigh pitting resistance. This adjusted composition results in the costof obtaining this steel being reduced, for there is no need to add otherelements, some of which are very expensive, such as tungsten, copper,magnesium, vanadium or rare earths such as cerium.

An important characteristic of the steel disclosed lies in that nocopper is added to the alloy, which has been possible due to the rest ofthe elements involved in its composition having been optimised withoutthereby losing the properties characteristic of steels of this kind.

More specifically, the stainless steel subject of the invention ischaracterised by the following composition:

C: not more than 0.02%

Mn: 0.65÷0.85%

Si: 0.40÷0.60%

Cr: 25.2÷25.6%

Ni: 6.2÷6.6%

Mo: 3.6÷3.8%

N: 0.24÷0.30%

P: not more than 0.025%

S: not more than 0.002%

B: 0.0015÷0.0030%

Ca: 0.0010/0.0050%

the rest of the alloy being obviously iron and other unintentionallyadded impurities. A steel with this composition not only has a goodmechanical strength (yield strength >600 Mpa), high pitting resistance(CPT >60°, as per ASTM G48) and a good intergranular corrosionbehaviour, but presents a high hot-formability as its maincharacteristic, resulting from the addition of boron and calcium.

The addition of considerable quantities of boron and calcium providesthe alloy subject hereof with a high hot-ductility, which makes itespecially suitable for a specific integrated manufacturing process,from steel meltdown to finally hot- or cold-forming a seamless tube witha diameter size of up to 240 mm.

Furthermore, the composition described for the alloy subject hereofallows good corrosion resistance properties to be obtained, and it isthus especially suitable to be used in corrosive environments, such assea water.

This alloy has been especially developed for obtaining seamless tubes,working a specific manufacturing process which includes the followingstages:

Meltdown in an electric-arc furnace

Refining in an AOD converter

Ascensional ingot casting

Gothic section billet rolling.

The billet obtained is then transformed into a seamless tube inaccordance with the following manufacturing stages:

Upsetting of the gothic billet into a round followed by hot piercing ina vertical press

Tube extrusion in a horizontal press

Reduction in a stretch reduction mill

Pilger cold-rolling.

Tests carried out with this type of austenitic-ferritic steel haveconfirmed that the tubes obtained in accordance with the aforesaidmanufacturing process have such good as mechanical strength, pittingresistance and hot-formability, which makes them especially suitable formaking seamless tubes of any diameter used in oil exploration in marineenvironments or for making equipment for the chemical or petrochemicalindustry.

We claim:
 1. Superduplex austenitic-ferritic steel applicable for makingseamless tubes with a high mechanical strength, high pitting resistance,a good intergranular corrosion behavior and a high hot-formability, in aprocess comprising the following steps: meltdown in an electric-arcfurnace, refining in an AOD converter, ascensional ingot casting andGothic section billet rolling, and then transforming the billet obtainedinto seamless tubes by upsetting the gothic billet into a round followedby hot piercing in a vertical press, tube extrusion in a horizontalpress, reduction in a stretch reduction mill and Pilger cold-rolling,essentially characterized in that the alloy is added standard elementssuch as C, Mn, Si, Cr, Ni, Mo, N, P, S with no Cu and W, and with theinclusion of significant quantities of B and Ca wherein the alloy has aweight percentage of C less than or equal to 0.02%, a weight percentageof Mn between 0.65% to 0.85%, a weight percentage of Si between 0.40% to0.60%, a weight percentage of Cr between 25.2% to 25.6%, a weightpercentage of Ni between 6.2% to 6.6%, a weight percentage of Mo between3.6% to 3.8%, a weight percentage of N between 0.24% to 0.30%, a weightpercentage of P less than or equal to 0.025%, a weight percentage of Sless than or equal to 0.002%, a weight percentage of B between 0.0015%to 0.0030%, and a weight percentage of Ca between 0.0010to 0.005%. 2.Superduplex austenitic-ferritic steel applicable for making seamlesstubes, as in all the preceding claims, wherein the resultant alloy has acomposition with the following weight percentages:C: not more than 0.02%Mn: 0.65 to 0.85% Si: 0.40 to 0.60% Cr: 25.2 to 25.6% Ni: 6.2 to 6.6%Mo: 3.6 to 3.8% N: 0.24 to 0.30% P: not more than 0.025% S: not morethan 0.002% B: 0.0015 to 0.0030% Ca: 0.0010 to 0.0050%the remainingweight percentage being Fe and other unintentionally added impurities.